The present invention relates to an apparatus for controlling a drive coil of a contractor in the holding mode.
An electronic drive control for a magnetic drive featuring pulse width modulation of the armature current is known from European Patent Application 0 789 378 A1.
In the case of pulse-width modulated signals, the problem exists that the TON-time cannot be graded, or stepped, with arbitrary fineness. In the holding mode, only {fraction (1/7)} to {fraction (1/12)} of the pick-up current is required, which results in the necessity of narrow pulses. In the case of the highest input voltages to be expected, the TON-time must even be considerably shorter, for example, 400 ns. Since only a graduation in finite steps of, for example, 100 ns can be attained, the ratio of the realizable steps to the required pulse duration is relatively high. A shortening of the mentioned shortest TON-time involves EMC problems and requires an outlay which is hardly justifiable any more. In this example, this already corresponds to an increment ratio of 25% and a holding power of 50%. In the case of high voltages, it is therefore not possible for the holding power to be minimized without limiting control, or actuating, reserves. This problem occurs, in particular, in the case of a DC voltage.
An object of the present invention is to provide an apparatus for controlling the drive coil of a contactor in a holding mode, which allows the holding power to be minimized in a simple manner even in the case of high voltages, without the occurrence of EMC problems and without limitation of the functional reliability.
The present invention provides an apparatus for electronically controlling a drive coil of a contactor in a holding mode using a pulse-width modulation. The apparatus includes an oscillator configured for generating an AC voltage, and a mixer configured for superposing a periodic output signal of the oscillator with an input variable associated with an input voltage so as to generate a pulse-width signal. An interval time for the pulse-width modulation is varied in time or the pulse width ratio is continuously changed.
Using the present invention, not only the holding power is maintained constant in a simple manner even in the case of high voltages but the EMC is improved as well.